Shock testing is used to characterize devices that are exposed to impact or shock in use. For example, electronic devices and components used in avionics applications such as gyroscopes and accelerometers can be exposed to shock in the form of high g-forces. Because of this, it is often desirable to characterize the effect of such high g-forces on such devices as part of the design process.
One shock testing technique for applying a high g-force shock to a test specimen is disclosed in U.S. Pat. No. 6,655,190 to Grossman et al., which is fully incorporated herein by reference for all purposes. The device and method of Grossman et al. uses a test system that includes an I-beam fixed at one or both ends. A test specimen is mounted to the beam and electrically connected to a monitoring system to record or monitor the effects of the high g-force shock provided to the test specimen during the test. The beam is loaded within the elastic range of the beam material by applying a force that deflects the beam away from a static position by a predetermined amount. Such loading creates a high strain on the beam and stores elastic energy in the beam. The high g-force event is created by suddenly releasing the stored energy in the beam by removing the force. A shockwave travels along a length of the test beam and applies a high g-force to the test specimen.
In order to create a high g-force in this way, it is desirable to release the loading of the beam suddenly and reliably. Releasing the beam suddenly contributes to maximizing the g-force that can be achieved for a particular beam and loading condition. This is because the length of time that it takes to completely remove the loading force from the beam can contribute to a decrease in the elastic energy that creates the high g-force event. Thus, when a sudden release is achieved, more of the elastic energy that is stored in the beam can efficiently contribute to the high g-force event.
Releasing the beam in a reliable manner relates to a desire to provide consistency and repeatability in testing. For certain test specimens, such as inertial devices for example, it is desirable to minimize any forces that might act on the beam from directions other than the desired or primary direction of the applied g-force. In order to do this, the force applied to the beam needs to be repeatedly released in a controlled and reliable manner. The need to have a clean and repeatable test is important for most test devices. If the test is repeatable then the relationship between the motions in various directions is fairly consistent. By monitoring motion in the primary direction there is reasonable confidence that the motion in the other directions stays within some limits. If the motion is primarily in one direction it is easier to determine the cause and effect relationship between the shock applied and the resulting behavior of the test specimen (breakage, shift in performance, etc.). Also, in order to be able to meaningfully compare test results from plural test runs, consistency throughout the test runs is desired. Reliability in testing is also desired because test specimens can be expensive or their availability may be limited. As such, it is undesirable for a test to fail. Moreover, shocking a test specimen with a g-force that is lower than the desired test force can make the test specimen unsuitable for further testing. Thus, for many applications, test specimens must be tested at the desired g-force on the first test run.
The Grossman et al. test device uses a hydraulic ram to apply the loading force to the beam. In particular, the ram applies the loading force to a ceramic column that is positioned between the ram and the beam. In this condition, the ceramic column is put in compression and supports the complete load directed to the beam. In order to conduct the test and cause a sudden beam release and thus create the high g-force event, the ceramic column is impacted with a projectile to fracture the ceramic column and quickly release the beam, thereby creating a high g-force event. This technique effectively releases the beam in a sudden and reliable manner. However, where it is desired to increase the g-force created by the test, an increased load is placed on the beam by the hydraulic ram through the ceramic column to provide greater stored elastic energy in the beam. In order to support the increased load, the diameter of the ceramic column is increased. The ceramic column thus experiences a correspondingly increased load and greater energy is required to be provided by the projectile to fracture the ceramic column because of its increased diameter. Where the ability to provide a projectile with sufficient energy is unavailable or difficult, other techniques that can suddenly and reliably fracture the ceramic column are desired.
After the force is removed from the beam and the high g-force event is completed, the beam will resonate until the energy stored in the beam is fully dissipated. The magnitude of the g-force is greatest during the first resonant cycle of the beam and it is the reaction of a test specimen to this maximum g-force that is usually of most interest. Subsequent resonance of the beam may be undesirable for certain applications. Such ringing can be undesirable where the purpose of a test is to simulate a real world shock event such as a gun launch or the like as accurately as possible. Repeated oscillations after the primary shock event are generally unrealistic and could conceivable cause damage to a test specimen that would not occur as a result of an actual real world shock event. Also, by eliminating the ringing it is easier to demonstrate by analysis of the data that a real world shock event has been adequately simulated.